Charge pumps are designed to change an input signal at one voltage to an output signal at a lower or higher voltage. This voltage change may enable a single voltage of a power supply to be provided at varied voltages depending upon the circuit. For example, a power supply of a device may provide a voltage of 1.5 volts (v); however, a circuit in the device may need an operating voltage of −1.5 v. In order to provide this operating voltage the charge pump may take the 1.5 v provided by the power supply and convert it to the −1.5 v needed by the circuit.
A common method of converting the voltage is to utilize a switched mode power supply with switches that open and close at a specific clock signal frequency. This clock signal frequency may impact the output of the charge pump in a couple of ways. First, when a voltage is applied to a charge pump, it takes a period of time before the output voltage of the charge pump is capable of being used by the circuit to which the voltage is being provided. This period of time is known as the settling time of the charge pump and is dependent upon the frequency of the clock signal. A second issue involves spurious emissions. Spurious emissions are unintended frequencies that may be introduced into the output signal of the charge pump that were not present in the input signal. Spurious emissions, like settling time, are also dependent upon the clock signal frequency.